Source: worked in ed covering this subject, studied assessment in ed school with one of the creators of the standards.
If you don't even want to be successful and/or wealthy, and you don't try, your chance of becoming so is definitely much lower.
The more you prepare specifically for a test, the harder it is to determine how much of your score is based on your general understanding of what is being tested, and how much is based on you exploiting the test.
If I wanted to know how well the average person on the street can clean a toilet and my test involved asking a bunch of people for a demonstration without pay nor feedback of any kind I don't suspect I would find a lot of clean toilets. I would instead have learned that people don't like cleaning toilets and gained no insight into their ability to do so.
If I dont study for the test, then you basically measure how long ago it was when I was revising the material last time (if it was last week I get much better result then if it was month and half ago) and you have even more random in terms of whether my learning coincided with I focused on what would count as "exploitation" of this test or different test. For example, was you focusing on speed during easy exercises or ability to solve hard problems last three weeks? It is pretty much random choice and preparation lets you get back to other mode relatively fast - while without it you get worst results with kids who accidentally did wrong thing and it has nothing to do with understanding of material, just a random chance.
Motivation to actually prepare yourself removes that random.
The exploitation of specific test, especially if you dont understand base material, gets you only so far compared to that.